In geometry class, the girl to boy ratio is $10$ to $5$. If there are $40$ girls, how many boys are there?
Answer: A ratio of $10$ girls to $5$ boys means that there is a group of $10$ girls for every group of $5$ boys. If there are $40$ girls, then there are $4$ groups of $10$ girls each. Because we know the ratio is $10:5$ , there must be $4$ groups of $5$ boys each. There is a total of $20$ boys in geometry class.